The present invention is directed to a rotary fastener. More particularly, the present invention is directed to a sheet metal screw for attaching two thin panels together, which screw will exhibit significantly increased stripping torque over other known sheet metal screws. With presently available sheet metal screws, the range between drive torque and strip torque is unacceptably small. For this reason, it is virtually impossible, given dimensional tolerances and variances in material, to find a single torque setting on power screw drivers which will drive the hardest to drive screw without stripping the most strippable screw. Such a situation results in a relatively high number of screws being stripped during installation which results in reduced clamp load between the two panels being secured or requires substantial operator time for removal and replacement of stripped screws. The screw of the present invention reduces the above-described problem by increasing the range between drive torque and strip torque of the screw. The sheet metal screw of the present invention has a recess in the lower side of the head surrounding the upper region of the shank and within this recess and for distance beyond the shank has formed thereon a helical ramp which may form an extension of the thread on the shank. In this regard, this screw is similar to the sheet metal screw disclosed in Applicant's pending application Ser. No. 316,260, filed Oct. 29, 1981 as a continuation-in-part of application Ser. No. 276,228, filed June 22, 1981. Additional features of the screw of the present invention include nibs on the underside of the head of the screw which traverse the outer boundary of the recess on the underside of the head of the screw. Further, the screw of the present invention includes a plurality of helical ribs, the two ends of which terminate at the outer boundary of the recess and the shank of the fastener, respectively. The height of the helical ribs decreases as the helical rib progresses from the outer boundary of the recess to the shank; the outer boundary termination point of the helical rib may coincide with an end of a nib. The purpose of the various arrangements of nibs and helical ribs disclosed herein is to "rake" up material as the underside of the head encounters the top of the first panel as the fastener is driven home and thereby provide further material to be drawn into the recess in the underside of the head to bind with the helical ramp or ramps on the shank of the fastener and, thereby, significantly increase the stripping torque of the fastener.
Other characteristics, features, and advantages of the present invention will become apparent after a reading of the following specification.